A common gas engine includes a spark plug disposed inside a combustion chamber, and flame-propagating combustion is performed starting from ignition of fuel gas by the spark plug. However, if lean-premix combustion is to be performed, or if a bore diameter is large (e.g. approximately 150 mm), it may be difficult to perform flame-propagating combustion in a limited time by ignition using a spark plug, and thus combustion efficiency may not be improved.
As a countermeasure to the above, a plurality of plugs may be provided, or a pre-combustion-chamber type gas engine may be employed.
A pre-combustion-chamber type gas engine is used for a gas engine which has a large bore diameter and which performs lean-premix combustion. The combustion process of the pre-combustion-chamber type gas engine is as follows.
(1) Fuel gas or rich air-fuel mixture is supplied to a pre-combustion chamber from a supply line of ignition fuel gas.
(2) In a compression stroke, lean premixed gas in a main combustion chamber flows into the pre-combustion chamber through a nozzle of the pre-combustion chamber to be mixed with the fuel gas or the rich air-fuel mixture in the pre-combustion chamber, and a theoretical combustion mixture ratio is achieved immediately before ignition.
(3) A spark plug sparks in the pre-combustion chamber and flame propagation combustion occurs.
(4) Combustion gas flows out to the main combustion chamber through the nozzle of the pre-combustion chamber in form of a torch.
(5) Torch combustion and flame propagating combustion occur in the main combustion chamber.
According to the above combustion process, low emission can be achieved because lean premixed gas is in the main combustion chamber. Further, even with lean premixed gas in the main combustion chamber, combustion can be completed in the main combustion chamber at a sufficient speed because a torch flame with enough energy is injected from the pre-combustion chamber.
Ignition fuel gas can be supplied to the pre-combustion chamber through the following two approaches, for instance:
(A) Provide a valve opened and closed by a cam in a gas supply channel for pre-combustion chamber for supplying ignition fuel gas, and supply fuel gas at a timing and a lift amount determined in advance.
(B) Provide a check valve in the gas supply channel for pre-combustion chamber, and supply ignition fuel gas at a certain timing corresponding to a differential pressure between a supply pressure (cylinder pressure) of fuel gas supplied from the main combustion chamber to the pre-combustion chamber and a pressure inside the pre-combustion chamber.
The approach (B) is more advantageous than the approach (A) in that the facility can be reduced in size and cost.
Patent Document 1 discloses a pre-combustion-chamber type gas engine with a pre-combustion chamber equipped with a spark plug and an ignition-fuel-gas channel that has an opening into the pre-combustion chamber. The pre-combustion-chamber type gas engine includes a throat-shaped channel or a ledge-shaped projection for narrowing a transverse plane of the pre-combustion chamber before the spark plug disposed in the pre-combustion chamber. With this configuration, rich air-fuel mixture with low flux is formed in the vicinity of the spark plug, which makes it possible to secure safe ignition.
Patent Document 2 discloses a pre-combustion-chamber type gas engine including a gas supply channel for pre-combustion chamber equipped with a check valve. The check valve opens and closes in accordance with a differential pressure between a channel upstream pressure and a pressure inside a pre-combustion chamber (cylinder pressure) as described above, and more specifically, opens when the pressure inside the pre-combustion chamber decreases to a predetermined value.